The mechanical properties of nanostructures and thin films are important in a number of potential applications such as memory devices, mass sensors, electrochemical transistors, oscillators and nanogenerators. However, measuring the mechanical properties of nanostructures and thin films is difficult for a variety of reasons. First, the mechanical properties of nanostructures cannot be extrapolated from bulk values. Material properties are partly dictated by their physical dimensions, notably the increased surface-to-volume ratio for small volumes. Material properties are also affected significantly by fabrication processes and are sensitive to the influence of interfaces and adjoining materials.
Also, due to their small physical dimensions, the well-established testing techniques used for evaluating the properties of bulk materials are inadequate for nanostructures. Tensile and creep testing of fiber-like materials require that the size of the sample be sufficiently large to be clamped rigidly by a sample holder without sliding. Such an approach is not applicable to nanostructures. Similarly, optical measurements commonly used to evaluate microelectromechanical systems (MEMS) are not valid for measuring the mechanical properties of individual nanowires because the diameters of nanowires are less than the wavelength of visible light. Additionally, the ultra-small size of the nanostructures makes their manipulation difficult and specialized techniques are necessary to pick up and weld individual nanostructures. Therefore, new methods and methodologies have to be developed to quantify the properties of those nanostructures.
In attempts to address these issues, various techniques have been developed to measure the properties of nanostructures. Among them, scanning probe microscopy techniques have been proven to be applicable approaches. One of the first studies regarding scanning probe microscopy measurement techniques was performed by Wong et al. These experiments provide experimental evidence that the mechanical properties of nanostructures may be inherently different from that of their bulk form. However, the experimental measurements have uncertainties, such as precise measurements of the thermal vibrational amplitudes, the effect of a measurement probe tip on the nanostructures, the magnitude of a friction force between the nanostructure and its substrate during bending, and calibration of a probe cantilever. In addition, the experiments did not provide information about the morphologies of stressed nanostructures or on the possible presence and/or evolution of defects trapped inside of the nanostructures.
Due to intrinsically simple geometry, quantitative uniaxial tensile tests on nanostructures have also attracted considerable attention from both theorists and experimentalists. Unlike experimental studies, in which a top-down approach is employed, computer simulations adopt a bottom-up approach to study the mechanical behavior of nanostructures. Such computer simulations have revealed several unexpected physical phenomenon including: (1) ultrahigh elastic strain and, therefore, ultrahigh yield stress; (2) crystalline-to-amorphous transitions; (3) increasing Young's modulus with decreasing cross-sectional area; and (4) crystal structure transition accompanying dramatic changes in Young's modulus. However, without experimental verification, such computer simulations should be regarded only as a source of inspiration and qualitative guidance.
With the high spatial resolution provided by transmission electron microcopy (TEM) and the small probed volume, a quantitative TEM tensile test apparatus provides an experimental means to directly measure the mechanical properties of nanostructures and thin films. Moreover, a quantitative TEM tensile test apparatus provides an opportunity to fill the gap between experiments and simulation. Also, in comparison to other quantitative TEM deformation techniques, such as the quantitative TEM indentation devices developed by Hysitron Inc., for example, TEM tensile tests take advantage of a simple geometry and, as a result, provide experimental results that are relatively easy to explain. A quantitative TEM tensile test device can measure elongation properties of thin films and can reveal the unique deformation mechanisms of nanocrystalline materials, which are known to have asymmetrical responses for compression and tensile tests.
Despite great promise, only a few TEM tensile test apparatuses are commercially available, none of which are truly quantitative. As qualitative investigation tools, products such as the TEM tensile holder from Gatan, Inc. can provide physical insight into how materials respond to an applied stress. However, the Gatan holder has several drawbacks which limit its application. First, a force sensor is not available. Second, although equipped with a digital reader for displacement at a micrometer resolution, a manually controlled motorized drive makes it extremely difficult to control the strain rate. Additionally, the Gatan holder has a minimum displacement step at the micrometer level, which makes it difficult to record clear images when shifting a sample from its original position. Also, the Gatan holder design requires at least two steps for sample preparation, the first of which being to make the area of interest of the sample electron transparent, and the second being to mechanically fix the sample to the holder. For thin film or high aspect ratio nanostructures, premature specimen failure during transfer and mounting often makes the test difficult. As such, a tensile test holder design including integrated force and displacement sensors and requiring only single-location sample preparation is desirable.
Some in-situ TEM tensile test holders have been developed by academic researchers. For example, a MEMS-based in-situ TEM tensile tester uses spring displacement to estimate applied force. Although the design provides encouraging information for the development of quantitative TEM nanomechanical testing, it does not allow recording of an applied force in realtime since the displacement measurement is based on an associated TEM image. The TEM image must also include the displacement measurement structure which adversely affects high resolution sample imaging due to its requirement of a large field of view.
Another device includes two types of actuators: a comb drive electrostatic actuator, which is force controlled, and an in-plane thermal actuator, which is displacement controlled. The Zhu and Espinosa device is capable of applying and measuring load independent of imaging. However, the approach directly welds samples rigidly to the sensor. Considering the practical difficulty in cleaning the residual parts after the test, a new sensor may be required after each test. Furthermore, although chips can be fabricated in large quantities, the calibration, especially with high accuracy and precision, can be difficult.